Magnetized No?i- crystalline Substances. c 2k5 



distance between the poles of the magnet ; and is thus ex- 

 plained by the discoverer himself. (§ 2269) * The cause of 

 the pointing of the bar, or any oblong arrangement of the 

 heavy glass, is now evident. It is merely a result of the ten- 

 dency of the particles to move outwards, or into the positions 

 of weakest magnetic action. * The joint exertion of the action 

 of all the particles brings the mass into the position which, by 

 experiment, is found to belong to it/ " 



8. It may be added to this, that the tendency of a bar, 

 whether of ferromagnetic or of diamagnetic substance, in a 

 uniform field of magnetic force, to take the direction of the 

 lines of force, depends on the effect of the mutual action of the 

 parts in altering the general magnetization of the bar, and is 

 consequently so excessively feeble for any known diamagnetic 

 substance that the most delicate experiments would in all pro- 

 bability fail to render it sensiblef . 



9. Faraday's law r , stated at the commencement of these re- 

 marksj may be illustrated by some very curious although 

 extremely simple experiments, which I shall now describe 

 briefly:}:. 



10. The special apparatus required is merely a long light 

 arm (I have used one about four feet in height; but a much 

 shorter rod, if suspended by a finer or by a longer torsion- 

 thread, would have answered equally well) suspended from a 

 " torsion-head " by means of a very fine wire, or thread of 

 unspun silk fibres attached to it near its middle; and a case 

 round it adapted to prevent currents of air from disturbing its 

 equilibrium, but allowing it sufficient angular motion in a ho- 

 rizontal plane. A small ball of soft iron is attached to one 

 end of the arm (or hung from it by a fine thread, which, for 

 the sake of stability in many of the experiments, as for in- 

 stance, experiments 2 and 3 described below, must not be too 

 long), and a counterbalance is adjusted near the other end so 

 as to make the arm horizontal.* If only a small angular mo- 

 tion be allowed to the arm, the path of the ball will be sen- 

 sibly straight, and we may consider that, by the arrangement 



* * The extreme feebleness of the diamagnetic action, on account of 

 which any small sphere or cube of the matter will experience very nearly 

 the same force as if all the rest were removed, seems fully to justify this 

 explanation.' 



-f A very brief communication on this subject was laid before the British 

 Association at the meeting of 1848, and is published in the Report for that 

 year, under the title " On the Equilibrium of Magnetic or Diamagnetic 

 Bodies of any form, under the Influence of Terrestrial Magnetic Force." 



X These experiments were shown, in illustration of lectures on magnet- 

 ism in the Natural Philosophy Class in the University of Glasgow, during 

 the Session 1848-49. 



